1. Field of the Invention
The present invention relates to a sealing material for optical elements capable of producing cured products which is excellent in transparency, crack resistance, and heat resistance, comprising a polysilsesquioxane compound as a major component, and to a sealed optical element comprising an optical element sealed by the cured product of the sealing material.
2. Description of Related Art
There are various types of optical elements including various lasers such as a semiconductor laser, light emitting devices such as a light emitting diode (LED), a photo acceptance unit, a composite light element, an optical integrated circuit, and the like. In recent years, an optical element of a blue light or a white light of which the peak wavelength of luminescence is short has been developed and is widely used. A rapid increase in brightness of light emitting devices with a short peak wavelength of luminescence tends to accompany an increase of the calorific power of the optical element.
An optical element is usually used as a sealed optical element in which the optical element is sealed with a cured product of a sealing material.
In general, a composition containing a transparent epoxy resin as a major component which can produce a cured product with excellent heat resistance and the like has been known as the sealing material for optical elements.
However, along with the increase in the brightness of optical elements in recent years, the cured products of the optical element sealing material are exposed to a high energy light and a high temperature generated by the optical elements, causing deterioration, which results in cracking and yellowing (coloration) of the cured products. Cracks produced in the cured sealing material make the optical elements unusable. The coloration reduces transparency and lowers the brightness of the optical elements.
In order to solve these problems, sealing materials for optical elements using a polysilsesquioxane compound as a major component have been proposed in Patent Documents 1 to 3. The polysilsesquioxane compounds disclosed in the Patent Documents 1 to 3 are claimed to produce a sealed optical element excelling in crack resistance and heat resistance.
However, it is sometimes difficult for the cured sealing materials for optical elements containing the polysilsesquioxane compounds disclosed in the Patent Documents 1 to 3 to sufficiently prevent yellowing and cracking when exposed to a high energy light or a high temperature generated by the optical elements. Therefore, development of a sealing material for optical elements which can produce a cured product exhibiting more excellent crack resistance and heat resistance has been desired.    [Patent Document 1] JP-A-2004-359933    [Patent Document 2] JP-A-2005-263869    [Patent Document 3] JP-A-2006-328231
The present invention has been achieved in view of this situation in commonly used technology and has an object of providing a sealing material for optical elements capable of producing a cured product which is excellent in crack resistance and heat resistance and is free from coloration or deterioration when exposed to high energy or a high temperature, and a sealed optical element comprising an optical element sealed by the cured product of such a sealing material.